Spin Transfer MRAM Device with Reduced Coefficient of MTJ Resistance Variation

ABSTRACT

We describe the manufacturing process for and structure of a CPP MTJ MRAM unit cell that utilizes transfer of spin angular momentum as a mechanism for changing the magnetic moment direction of a free layer. The cell is formed of a vertically or horizontally series connected sequence of N sub-cells, each sub-cell being an identical MTJ element. A statistical population of such multiple sub-cell unit cells has a variation of resistance that is less by a factor of N −1/2  than that of a population of single sub-cells. As a result, such unit cells have an improved read margin while not requiring an increase in the critical switching current.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a current perpendicular to planerandom access memory (CPP-MRAM) cell formed as a magnetic tunnelingjunction (MTJ) and using a spin transfer effect with enhanced spintorque.

2. Description of the Related Art

The conventional magnetic tunneling junction (MTJ) device is a form ofultra-high magnetoresistive device in which the relative orientation ofthe magnetic moments of parallel, vertically separated, upper and lowermagnetized layers controls the flow of spin-polarized electronstunneling through a very thin dielectric layer (the tunneling barrierlayer) formed between those layers. When injected electrons pass throughthe upper layer they are spin polarized by interaction with the magneticmoment of that layer. The majority of the electrons emerge polarized inthe direction of the magnetic moment of the upper layer, the minoritybeing polarized opposite to that direction. The probability of such apolarized electron then tunneling through the intervening tunnelingbarrier layer into the lower layer then depends on the availability ofstates within the lower layer that the tunneling electron can occupy.This number, in turn, depends on the magnetization direction of thelower electrode. The tunneling probability is thereby spin dependent andthe magnitude of the current (tunneling probability times number ofelectrons impinging on the barrier layer) depends upon the relativeorientation of the magnetizations of magnetic layers above and below thebarrier layer. The MTJ device can therefore be viewed as a kind ofmulti-state resistor, since different relative orientations (e.g.parallel and antiparallel) of the magnetic moments will change themagnitude of a current passing through the device. In a common type ofdevice configuration (spin filter), one of the magnetic layers has itsmagnetic moment fixed in direction (pinned) by exchange coupling to anantiferromagnetic layer, while the other magnetic layer has its magneticmoment free to move (the free layer). The magnetic moment of the freelayer is then made to switch its direction from being parallel to thatof the pinned layer, whereupon the tunneling current is large, to beingantiparallel to the pinned layer, whereupon the tunneling current issmall. Thus, the device is effectively a two-state resistor. Theswitching of the free layer moment direction (writing) is accomplishedby external magnetic fields that are the result of currents passingthrough conducting lines adjacent to the cell.

FIG. 1 is a highly schematic drawing showing an overhead view of aconventional MRAM cell between orthogonal word (200) and bit (100)lines. The cell (1000) is drawn with a slightly elliptical horizontalcross-section because such a shape produces a magnetic anisotropy withinthe free layer that assists its magnetic moment in retaining a thermallystable fixed position after switching fields have been turned off. Thefields produced by currents in each of the two lines are between about30 to 60 Oersteds in magnitude. According to the diagram, the word linefield will be along the hard axis of the cell, the bit line field willbe along the easy axis (the longer axis of the ellipse).

The use of magnetic fields externally generated by current carryinglines (as in FIG. 1) to switch the magnetic moment directions becomesproblematic as the size of the MRAM cells decreases and, along withtheir decrease, so does the width of the current carrying lines. Thesmaller width lines require greater current densities to produce thenecessary switching fields on the MTJ elements, greatly increasing powerconsumption.

For this reason, a new type of magnetic device, called a spin transferdevice and described by Slonczewski, (U.S. Pat. No. 5,695,164) and byRedon et al. (U.S. Pat. No. 6,532,164) has been developed and seems toeliminate some of the problems associated with the excessive powerconsumption necessitated by external switching fields. The spin transferdevice shares some of the operational features of the conventional MTJcell described above, except that the switching of the free layermagnetic moment is produced by the spin polarized current itself. Inthis device, unpolarized conduction electrons passing through a firstmagnetic layer having its magnetic moment oriented in a given direction(such as the pinned layer) are preferentially polarized by their passagethrough that layer by a quantum mechanical exchange interaction with thepolarized bound electrons in the layer. Such a polarization can occur toconduction electrons that reflect from the surface of the magnetizedlayer as well as to those that pass through it. When such a stream ofpolarized conduction electrons subsequently pass through a secondmagnetic layer whose polarization direction is not fixed in space (suchas the free layer), the polarized conduction electrons exert a torque onthe bound electrons in the magnetic layers which, if sufficient, canreverse the polarization of the bound electrons and, thereby, reversethe magnetic moment of the magnetic layer. The use of a spin-polarizedcurrent internal to the cell to cause the magnetic moment reversalrequires much smaller currents than those required to produce anexternal magnetic field from adjacent current carrying lines to producethe moment switching. Recent experimental data (W. H. Rippard et al.,Phys. Rev. Lett., 92, (2004)) confirm magnetic moment transfer as asource of magnetic excitation and, subsequently, magnetic momentswitching. These experiments confirm earlier theoretical predictions (J.C. Slonczewski, J. Magn. Mater. 159 (1996) LI, and J. Z. Sun, Phys. Rev.B., Vol. 62 (2000) 570). These latter papers show that the net torque,Γ, on the magnetization of a free magnetic layer produced byspin-transfer. from a spin-polarized DC current is proportional to:

Γ=sn _(m)×(n _(s) ×n _(m)),  (1)

Where s is the spin-angular momentum deposition rate, n_(s) is a unitvector whose direction is that of the initial spin direction of thecurrent and n_(m) is a unit vector whose direction is that of the freelayer magnetization and x symbolizes a vector cross product. Accordingequation (1), the torque is maximum when n_(s) is orthogonal to n_(m).

Referring to FIG. 2, there is shown a schematic illustration of anexemplary prior art MTJ element being contacted from above by a bit line(100) and from below by a bottom electrode (200). Moving verticallyupward, there is shown a seed layer (1), an antiferromagnetic pinninglayer (2), a synthetic antiferromagnetic (SyAF) pinned reference layer(345), consisting of a first ferromagnetic layer (3), a non-magneticspacer layer (4) and a second ferromagnetic layer (5), a non-conductingtunneling barrier layer (6), a ferromagnetic free layer (7) and anon-magnetic capping layer (8). Arrows, (20) and (30), indicate theantiparallel magnetization directions of the two ferromagnetic layers(3) and (5) of the SyAF pinned layer (345). The double-headed arrow (40)in layer 7 indicates that this layer is free to have its magnetic momentdirected in either of two directions.

Referring again to FIG. 2 it is noted that when a critical current(arrow (50) is directed from bottom to top (layer (1) to layer (8)), thefree layer magnetization (40) would be switched to be opposite to thedirection of the reference layer's magnetization (30) by thespin-transfer torque. This puts the MTJ cell into its high resistancestate.

Conversely, if the current is directed from top to bottom, the freelayer magnetization (40) would be switched, by torque transfer ofangular momentum, to the same direction as that of the pinned referencelayer (30), since the conduction electrons have passed through thatlayer before entering the free layer. The MTJ element is then in its lowresistance state.

Referring again to FIG. 2, there is shown some additional circuitry,specifically a transistor (500) to inject current into the cell elementwhenever the cell element is selected to be written upon. The transistoris electrically connected to the cell through a conducting via (80)which allows a current to pass vertically between the bottom electrode(300) and the bit line (100). The word line (200), which can contact thetransistor gate activates the transistor so as to inject the writingcurrent. In this way one can create a single spin-RAM memory cell thatutilizes the spin transfer effect (denoted hereinafter as an STT-RAM)for switching an MTJ type element. In this paper, we will use the term“element” to describe the basic MTJ structure comprising a tunnelingbarrier layer sandwiched between ferromagnetic fixed and free layers. Weshall use the term “memory cell” to denote the combination of the MTJelement incorporated within circuitry that permits the element to bewritten on and read from. The word line provides the bit selection(i.e., selects the particular cell which will be switched by means of acurrent passing through it between the bit line and the source line) andthe transistor provides the current necessary for switching the MTJ freelayer of the selected cell. Although it is not shown in this simplifiedfigure, the cell is read by applying a bias voltage between the bit lineand source line, thereby measuring its resistance and comparing thatresistance with a standard cell in the circuit. It is to be noted thatlarge cell arrays are subject to difficulties that arise fromstatistical variations in the magnetic and electrical properties of eachcell. For example, to decide whether a cell is in its high or lowresistance state, its resistance must be compared to that of a referencecell that is in a known resistance state. However, statisticalvariations in the high and low resistance values of the array cells andthe reference cells often make it possible to incorrectly interpret theresistance value of a cell. For this reason, the maximum variations ofthe resistance values of cells must fall within a “read margin” (oferror) so that a correct interpretation of a resistance value is made inall cases. In fact, it is the goal of the present invention to provide amethod of improving this read margin without sacrificing any of thefeatures of the memory cell.

The critical current for spin transfer switching, I_(c), is generally afew milliamperes for a 180 nm sub-micron MTJ cell (of cross-sectionalarea A approximately A=200 nm×400 nm). The corresponding criticalcurrent density, J_(c), which is I_(c)/A, is on the order of several 10⁷Amperes/cm². This high current density, which is required to induce thespin transfer effect, could destroy the insulating tunneling barrier inthe MTJ cell, such as a layer of AlOx, MgO, etc.

During the reading of data, a small current flows across the MTJ celland its resistance is compared with a pre-written MTJ cell (not shown)called a reference cell, to determine whether the cell being read is ina high or low resistance state. Typically, the reading margin isdetermined by the ratio between the magneto-resistive ratio, dR/R (thedifference between the maximum and minimum resistance of the celldivided by its maximum resistance) and the coefficient of resistancevariance, σ/μ, (the ratio between resistance standard deviation σ andresistance mean value μ).

Normally, the write current density required to switch the direction ofthe free layer magnetization is mainly determined by the free layermagnetic moment, damping ratio and spin-angular momentum depositionrate, which depend on the MTJ film materials and their quality. As theMTJ device is microminiaturized to nanometer scale dimensions, the writecurrent density is unchanged, giving a much smaller write current whichis scalable to the shrinking MTJ cell dimensions. Hence, powerconsumption in the device is reduced.

However, as the MTJ cell dimensions become smaller and smaller, the MTJresistance variation rapidly increases. For example. using the same MTJfilm materials and deposition processes, the coefficient of MTJresistance variance is found to be inversely proportional to the squareroot of each MTJ junction area. This makes the reading process verydifficult, even impossible, without a great increase in themagneto-resistive ratio dR/R. To address this problem, a spin transferMRAM structure with a special arrangement of MTJ cells is proposed toreduce the resistance variance.

Various combinations of MTJ cells can be found in the prior art. Huai etal. (U.S. Pat. No. 7,009,877) shows an MTJ element and a spin transferelement arranged vertically. In this invention they use an MTJ and aCPP-GMR immediately connected together to achieve a low write switchingcurrent. They also include the combination of two different MTJ cells,one with a smaller dR/R than the other, to achieve a low write switchingcurrent.

Hosotani (US Patent Application 2006/0221680) and Ju et al. (US PatentApplication 2006/0202244) and Nickel et at (US Patent Application2005/0195649) all disclose two MTJ elements connected in series and areused to write two bits per cell. In these applications two different MTJcells are connected together with different anisotropy directions intheir layers, obtained either by different shape orientations ordifferent magnetic materials.

Nguyen et al (U.S. Pat. No. 6,992,359) disclose a method for reducingwrite current density for spin transfer by using a free layer having ahigh perpendicular magnetic anisotropy. The purpose is to achieve a lowwrite switching current for a spin-transfer MRAM.

None of the above prior art discuss a method for reducing resistancevariation, which will help the reading process for a spin-transfer MRAM.To address this problem, we propose a spin-transfer MRAM cell structurewith a special arrangement of MTJ elements designed to reduce theresistance covariance. In this arrangement, electric current flowsacross two or more identical MTJ elements (denoted “sub-cells”)substantially identical in structure to the MTJ element shown in FIG. 2.The configuration will be described in greater detail with reference toFIGS. 3a and 3b below.

During the writing process, the required critical current for switchingthe magnetization direction of an MTJ free layer is the same for all MTJsub-cells connected in series. Thus, the same size current-supplyinglocal transistor is required as would be needed for writing a single MTJelement. During the reading process, if each individual sub-cell has amean resistance value R_(p) and if the resistance values are distributedwith a standard deviation σ, the mean value of the total resistance inone MTJ MRAM cell unit containing N sub-cells is the product (N)(R_(p)),while the standard deviation of total resistance of many such MRAM unitsis the product σ(N)^(1/2). Therefore, given that the coefficient ofresistance of one MTJ sub-cell is σ/R_(p), the coefficient of resistanceamong the MTJ MRAM units is the product (N)^(−1/2) (σ/R_(p)). In otherwords, the coefficient of resistance is reduced by a factor of(N)^(1/2), yielding a greatly increased reading margin.

SUMMARY OF THE INVENTION

A first object of this invention is to provide an MRAM deviceconfiguration that provides a greatly increased read margin while notrequiring any increase in critical current or local transistor size.

A second object of this invention is to provide a MRAM deviceconfiguration that includes a plurality of series-connected MTJsub-cells in which the coefficient of resistance of the configuration asa whole is reduced as compared with the coefficient of resistance of theindividual sub-cells.

These objects are achieved by an MRAM design in which N (where N isequal to or greater than 2) MTJ sub-cells of identical size areconnected in series so as to function as a single unit in a spintransfer MRAM structure. Thus, the passage of a critical currentsimultaneously switches the free layer magnetization of the free layerin each sub-cell. This arrangement requires the same write current orthe same local transistor size, yet it gives a greatly increased readingmargin as a result of a coefficient of resistance variation that isreduced by a factor of N^(−1/2). Thus, in a given unit cell of amulti-cell array, each unit cell will comprise two or more identicalsub-cells connected in series between a local transistor and a bit line.The sub-cells may be sequentially connected either in a horizontalconfiguration, in which adjacent series connected cells are at the samevertical level, or a vertical configuration in which sequentiallyadjacent cells are at different vertical levels. In each configuration,as will be shown in greater detail below, the bottom electrode of thefirst sub-cell is connected to the local transistor, the top electrodeof the last sub-cell is connected to the bit line and any adjacentintermediate sub-cells are connected by electrically contacting a topelectrode to a bottom electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a prior-art MTJ MRAM devicelocated at the junction of word and bit lines.

FIG. 2 is a schematic cross-sectional view of a typical prior artspin-transfer MRAM device formed using a single MTJ element.

FIG. 3a and FIG. 3b are schematic representations of two preferredembodiments of the present invention using a plurality of MTJ sub-cellsthat are connected either horizontally or vertically.

FIG. 4a-4f are schematic representations illustrating the fabricationsteps required to form an array of sub-cells such as described in FIG. 3b.

FIG. 5a -FIG. 5d are schematic representations illustrating thefabrication steps required to form an array of sub-cells such asdescribed in FIG. 3 a.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiment of the present invention is an MRAM device (asingle unit cell or an array of such cells) of the spin-transfervariety, having as the basic components of each unit cell CPP-MTJelements. These elements are configured as a series-connected sequenceof MTJ sub-cells so that the critical switching current is the same asfor a single cell yet the variation of resistance is significantlyreduced.

Referring to FIG. 3a , there is shown a first embodiment of thisinvention. In this embodiment a plurality of N sub-cells, where N is aninteger greater than 1 (only two (10), (11) being shown for simplicity)are formed in a horizontal series connected sequence. Looking downwardfrom the top of the diagram, there is shown a horizontally extending bitline (100). At the bottom of the diagram is shown (highly schematically)a current providing transistor (500). A word line (200) that extendsperpendicularly to the plane of the figure and, correspondingly,transverse to the bit line, substantially contacts the gate of thetransistor. A conducting via (600) is shown extending upward throughthree exemplary layers (610), (620), (630) of metallization (levels ofthe circuit integration along which electrical connections are made) toultimately contact the bottom electrode (300) of a first MTJ sub-cell(10). The first sub-cell has a top electrode (250) which extendshorizontally to electrically contact the bottom electrode (301) of asecond sub-cell (11). The contact from (250) to the bottom electrode(301) typically requires a conducting via (50). The second sub-cell (11)has a top electrode (251), that electrically contacts the bit line (100)by passing through a via (261), thereby completing the series circuitthrough the two sub-cells. If there were more than two sub-cells, itwould be the top electrode of the final sub-cell that would contact thebit line. In any given number of such horizontally connected sub-cells,the passage of a critical current from the transistor to the bit linewould simultaneously switch the magnetization direction of each freelayer in each sub-cell. The critical switching current of thisarrangement is the same as for a single cell, yet the coefficient ofresistance variation is reduced by a factor of 2^(1/2) (or N^(1/2) for Nsub-cells).

Referring now to FIG. 3b , there is shown a second embodiment of thisinvention in which the series connection of two exemplary sub-cells isin the vertical direction. As in FIG. 3a , there is shown a bit line(100) and a current providing transistor (500). A word line (200)extends out of the figure plane and contacts the gate of the transistor.A conducting via (600) extends vertically upward through three exemplarymetallization layers (610), (620), (630), whereupon it electricallycontacts a bottom electrode (300) of the first sub-cell (10). The topelectrode (250) of the first sub-cell serves as a bottom electrode of asecond sub-cell (11) and the top electrode (251) of the second cellelectrically contacts the bit line (100) to complete the circuit betweenthe transistor and the bit line. Like the first embodiment, the criticalswitching current of this arrangement is the same as for a single cell,yet the coefficient of resistance variation is reduced by a factor of2^(1/2) (or N^(1/2) for N sub-cells). In any given number of suchvertically connected sub-cells, the passage of a critical current fromthe transistor to the bit line would simultaneously switch themagnetization direction of each free layer in each sub-cell.

Referring now to FIG. 4a , there is shown, schematically, a typicalfabrication process of this second embodiment, in which the sub-cellsare series connected in a vertical configuration. In this process thereis first formed on a substrate (1000) a first bottom electrode (300) onwhich is formed a first MTJ element film stack (10) that willsubsequently be patterned to become the first sub-cell. The substratemay already contain a conducting via (600) that connects to a transistorin a lower layer of the fabrication. This via will not be shown insubsequent drawings but its presence is understood. Note, in thisterminology the film stack refers to the succession of verticallystacked layers that, when horizontally patterned, will become an MTJsub-cell. The vertical structure of the stack is the same as thevertical structure of the MTJ cell shown in FIG. 2, so, for clarity, thestack will be shown as a layer (10) without structure.

The horizontal extent of the film stack as well as the horizontal extentof the bottom electrode is sufficient to form, by lithographicpatterning to a desired horizontal cross-sectional area, a plurality ofsub-cells that will be connected to form a sequentially series connectedunit MRAM cell.

As indicated by the cell structure in FIG. 2, the film stack comprises,in vertical ascending order, a seed layer (1), an antiferromagneticpinning layer (2), a pinned layer (345), that is typically a synthetic(SyAP) structure comprising ferromagnetic layers (3) and (5) separatedby a non-magnetic coupling layer (4), a tunneling barrier layer (6), aferromagnetic free layer(7) and a capping layer (8). These layers areformed using conventional techniques that are known in the art.

Referring to FIG. 4b , there is shown the bottom electrode (300) andfilm stack (10) having been patterned by photolithography and etchingprocesses. In this process the film stack is provided with the desiredhorizontal cross-section, such as an elliptical cross-section. We shallretain the same numbering for the patterned stack and electrode as forthe unpatterned layers.

Referring to FIG. 4c , there is shown the patterned stack surrounded bya dielectric refilled layer (400) that has been planarized by, forexample, a CMP process, so that the top of the dielectric and the top ofthe patterned stack share a common co-planar surface (450).

Referring to FIG. 4d , there is shown the deposition of the secondbottom electrode (250), which is also the top electrode of the alreadydeposited sub-cell. A second film stack (11), identical to the firstfilm stack, is formed on the second bottom electrode.

Referring to FIG. 4e , there is shown the patterning of the second filmstack (11) followed by the deposition of a second dielectric refillinglayer (401) and its planarization to form a co-planar surface (451) withthe top of the second sub-cell (11) (the patterned stack).

Referring to FIG. 4f , there is now shown the deposition of the secondtop electrode (251), on surface (451) of the previous FIG. 4e , followedby deposition of a surrounding refill dielectric (402), itsplanarization, and the formation of a bit line (100) on the planarizedsurface.

It should be understood by those skilled in the art that the stepsoutlined above in FIG. 4a to FIG. 4f can be repeated in precisely thesame manner to form a succession of sub-cells of any desired number,with N sub-cells, where N is an integer greater than 1, denoting thegeneral fabrication size.

Referring now to FIG. 5a , there is shown a first step in a process thatcan fabricate the first embodiment of this invention, namely asuccession of linearly connected sub-cells formed in a horizontalconfiguration. Like the vertical configuration described in FIG. 4a ,the first step in the horizontal configuration is the deposition of abottom electrode layer (300) and a film stack (10).

Referring to FIG. 5b , there is shown the patterning of two horizontallyseparated sub-cells (10) and (11) on bottom electrodes (300) and (301)respectively. There is also shown the deposition and planarization of adielectric refill layer (400) to form planar surface (451) including theupper surfaces of cells (10) and (11). In the general case, Nhorizontally separated MTJ sub-cells and their bottom electrodes wouldbe formed, where N is an integer greater than 1.

Referring to FIG. 5c , there is shown the formation of a conducting via(50) through the planarized layer (400) and the formation and patterningof two top electrodes (250) and (251) over the planarized surface ((451)in FIG. 5b ). In the case of N MTJ sub-cells, the top electrode layerwould be patterned into N sections, with each of N−1 sectionselectrically connecting an MTJ sub-cell to the bottom electrode of aneighboring adjacent sub-cell through a conducting via.

Referring to FIG. 5c 1, there is shown the formation of a surroundingdielectric layer (600). The top surface of this layer will be planarizedto allow the formation of a bit line (100) and the top via (261) overthe top electrode (251) of the last (the Nth) MTJ sub-cell will bevertically extended so that it allows the top electrode (251) toelectrically contact by the bit line. It is understood that bottomelectrode (300) can contact via (800) shown so that contact to atransistor (not shown) can be made. This connection is shown in FIG. 3a.

As is finally understood by a person skilled in the art, the preferredembodiments of the present invention are illustrative of the presentinvention rather than limiting of the present invention. Revisions andmodifications may be made to methods, materials, structures anddimensions employed in forming and providing a CPP MTJ MRAM cell deviceusing transfer of spin angular momentum formed as a series connectedsequence of sub-cells, while still forming and providing such a deviceand its method of formation in accord with the spirit and scope of thepresent invention as defined by the appended claims.

What is claimed is:
 1. A spin transfer MRAM unit cell comprising: alocal transistor capable of providing a critical switching current to anMTJ cell element; a horizontally directed word line contacting saidtransistor and capable of activating said transistor so that a currentis produced; a horizontally directed bit line, vertically separated fromsaid word line and directed transversely to said word line; aconfiguration of N vertically adjacent MTJ sub-cells, wherein N is aninteger greater than 1, said configuration including a first sub-celland a last sub-cell and said configuration being electrically connectedin linear series, wherein all MTJ sub-cells have the same multi-layerstructure with the same geometry and each sub-cell comprises, in avertically stacked configuration, a bottom electrode, a pinning layer, asynthetic pinned layer, a tunneling barrier layer, a free layer and anupper electrode and wherein the bottom electrode of said first sub-cellof said N sub-cells electrically contacts said local transistor, and thetop electrode of said last sub-cell of said N sub-cells electricallycontacts said bit line, and wherein said top electrode and said bottomelectrode of each pair of vertically adjacent sub-cells are inelectrical contact, whereby a critical current capable of simultaneouslyswitching the magnetization of said free layer in each sub-cell can passvertically between said transistor and said bit line.
 2. The unit cellof claim 1 wherein the resistance variations of a statistical populationof said unit cell is less by a factor of N^(−1/2) than the resistancevariations of a statistical population of a unit cell having equivalentproperties and formed of a single sub-cell.